This invention relates to MOSgated semiconductor devices and more specifically relates to such devices using a silicon carbide (SiC) substrate and with improved gate dielectric breakdown characteristics and increased channel mobility.
It is well known to use silicon carbide as a substrate material for power semiconductor devices. It is also known to use a trench structure for such devices (sometimes called UMOSFETs) in which an invertible channel is formed along the walls of such a trench.
It is known that silicon carbide UMOSFETs can have up to 200 times lower drift region resistance than monocrystaline silicon devices of the same rating. However, UMOSFETs fabricated on SiC have much lower breakdown voltages than is theoretically predicted. This is due to fact that the blocking performance of the SiC trench device is limited by gate dielectric breakdown usually at the trench corner. The highest blocking voltages observed to date on trench MOSFETs in SiC is 1400V (A. K. Agarwal et al., Int. Conf. on SiC, III-N and Related Mat. 1997, Stockholm, Sweden). Gate breakdown at the trench corner was improved by using a thick deposited oxide at the trench bottom and corner. Alternate structures such as Double Implanted MOSFETs (DIMOSFETs, J. N. Shenoy et al., Dev. Res. Conf. 1996), accumulation mode FETs (ACCUFETs, P. M. Shenoy et al., Int. Conf. on SiC, III-N and Related Mat. 1997, Stockholm, Sweden) and epitaxial channel FETs (EC-FETs, K. Hara, Int. Conf. on SiC, III-N and Related Mat. 1997, Stockholm, Sweden) have also been reported.
Silicon carbide comes in many poly types, predominantly "4H" and "6H", made by Cree Research Corp. The mobility of carriers in these 4H and 6H substrates is directionally dependent. Both the lateral and vertical bulk electron mobility of 4H material is relatively high in each direction (about 800 cm.sup.2 /V.s in the lateral direction and about 1000 cm.sup.2 /V.s in the vertical direction) whereas the 6H type has a lower lateral mobility (about 400 cm.sup.2 /V.s) and a substantially lower (about 80 cm.sup.2 /V.s) bulk electron mobility in the vertical direction. As a result 4H material is the substrate of choice for devices requiring vertical current flow.
It is also desirable to reduce premature breakdown of the device by breakdown of the gate oxide in the corner of the trench. It would further be desirable to create a depletion mode type MOSFET with a SiC substrate and a simplified manufacturing process.